In U.S. Pat. No. 5,020,091 there is described a cellular telephone communications system that includes multiple cells covering a geographical area, such as a metropolitan area. To the base station of each cell a number of frequencies are assigned, with some of the assigned frequencies being specified as control channels used for reciprocal identification between the telephones and the system. The location of the dedicated control channels in the cell's frequency spectrum identifies the type of cellular system, A or B (see, for example, EIA/TIA-553). A metropolitan area typically has one of each type of system.
A cellular radiotelephone operating in a cellular radiotelephone system that employs autonomous registration must make itself known to, or register with, the cellular system (i.e., with the MTSO). The identification typically informs the system where the radiotelephone is located in the system so that the radiotelephone can be subsequently paged when receiving a call.
In a cellular system of most interest to this invention the channel from a base station to the mobile station is referred to as the forward channel, i.e., the forward control channel and the forward voice channel. The channel from the mobile station to the base station is referred to as the reverse channel, i.e., the reverse control channel and the reverse voice channel.
The mobile station's transmitting channels are, by example, from 824.040 to 848.970 MHz and the receiving channels are, by example, from 869.040 MHz to 893.970 MHz. Further by example, the frequency 844.980 MHz is channel 666 for the mobile station's transmitter, and the frequency 889.980 MHz is channel 666 for the mobile station's receiver. When a system allocates a voice channel to the mobile station (e.g., channel 666), it allocates a frequency pair for receiving and transmitting. The frequency pair is referred to simply as channel 666. The receiving and transmitting frequencies of a given channel are always spaced apart by 45 MHz from one other.
Voice channels and access channels use both the forward and reverse channels. The system may have one or more forward control channels used for paging purposes only, and the mobile station is not allowed to transmit on the corresponding reverse channel (i.e., 45 MHz below the paging channel frequency). The forward paging channel is used only for receiving pages and orders from the base station.
However, in most cases the access channel set and the paging channel set are the same, and may comprise 21 channels (i.e., 42 frequencies). In this case the mobile station is allowed to transmit on the reverse control channels, while the corresponding forward control channels are used for paging and for sending orders to the mobile station. In this case the orders can also include messages from the base station to the mobile station, which are related to system access, e.g. registration confirmation or initial voice channel designation (in the case of a page response).
In greater detail, and referring to FIG. 5A, there is shown an exemplary mobile station transmit (TX or reverse) frequency band and an exemplary mobile station receive (RX or forward) frequency band. The depiction of FIG. 5A is based on the EIA/TIA-553 frequency band allocation (mandatory). Each band is divided into 666 frequency channels, with half of the frequency channels being assigned to the A system and the remaining half to the B system. Although not illustrated in FIG. 5A, it should be realized that additional or extended frequency bands, also described in EIA/TIA-553, could also be allocated. The total width of the mandatory band is 20 MHz+20 MHz. Each band includes a set of dedicated control channels (e.g., 21 control channels) for the A system, and a corresponding set of dedicated control channels for the B system. In the illustrated case there is a 45 MHz separation between the TX and RX channels. That is, the TX channel number 1 is 45 MHz below the RX channel 1, the TX channel number 2 is 45 MHz below the RX channel 2, etc. As such, it is useful to think of frequency channel pairs, wherein a given mobile station, when assigned to a voice channel (e.g., channel 22), would receive from the base station at a frequency of n MHz, and would transmit to the base station at a frequency of (n-45) MHz. Individual ones of the dedicated control channels are thus also a frequency channel pair. By example, dedicated control channel 1 of the B system (i.e., channel 334) is received by the mobile station from the base station at frequency n, and the mobile station transmits to the base station at frequency (n-45) MHz.
In most if not all current systems each dedicated control channel is actually a combined paging and access (P/A) channel, wherein paging messages and system access information are transmitted to the mobile station on an assigned forward dedicated control channel, and wherein the mobile station transmits access requests (e.g., call origination messages) and other information to the base station on the corresponding reverse channel of the P/A channel pair. This case is illustrated in FIG. 5B for the A system, wherein a plurality (e.g., 21) of P/A frequency channel pairs exist, individual ones of the P/A frequency channels being offset by 45 MHz from one another. In this case the set of dedicated P/A control channels may be considered to be congruent, wherein each forward P/A channel has an associated reverse P/A channel that is 45 MHz below the forward P/A channel.
It is also useful to consider the case of FIG. 5C, wherein the set of dedicated control channels may be considered to intersect or overlap. In this case only some of the dedicated control channels are P/A channels (i.e., the region of intersection of the set of paging channels with the set of access channels), while others are paging-only channels (P) and still others are access-only channels (A). This configuration could be used if, by example, it was required that the mobile station not transmit on a particular frequency within the reverse or TX band. By example, if for some reason it was required that no transmissions occur at a frequency f.sub.x, then a paging-only forward channel can be established at frequency (f.sub.x +45 MHz). A mobile station assigned to this paging channel will receive pages from the base station on the forward paging-only channel, will receive access information on a different forward access-only channel, and will transmit origination messages and the like on a reverse access channel that corresponds to (i.e., that is 45 MHz below) the assigned forward-only access channel (mobile stations do not transmit paging messages). In this case no mobile station will transmit at frequency f.sub.x, which is the desired result.
Still another case, not illustrated, provides a set of paging-only channels and a set of access-only channels. In this case the sets of paging and access channels can be considered to be disjoint or non-overlapping. This latter case is of least interest to the teaching of this invention.
The base stations frequently transmit an Overhead Message Train (OMT). The OMT includes a System Parameter Overhead Message (SPOM) that consists of two words. The SPOM always contains System Identification (SID) information. According to current standards (e.g., EIA/TIA-553), the SPOM is transmitted every 800.+-.300 ms. The OMT may contain additional words, such as the REGID, which in some systems is sent in, by example, every tenth OMT.
This system identification process, which takes place over a paging channel, is accomplished when the radiotelephone is powered on and receiving, but not transmitting (i.e., the radiotelephone is in an idle state).
Autonomous registration is a method by which the radiotelephone informs the system that it is active. One technique for performing autonomous registration is described in U.S. Pat. No. 4,775,999. This registration is performed at various time intervals. If the radiotelephone moves to another cellular communications system (with a corresponding change in the SID) it must register in the new system.
As is depicted in FIG. 1, a typical cellular network is divided into a number of regions (e.g., SID1-SID5) each having contiguous radio cells (shown nominally as hexagons). One region or SID may also include one or more paging areas, each encompassing one or more radio cells. One advantage of having several paging areas in one SID is that when a call to a mobile station is received in the servicing system, the paging message to the called mobile station can be transmitted to only one paging area (where the mobile station is assumed to be located), instead of being transmitted over the entire coverage of the SID. Dividing the SID into several paging areas thus enables an increase in the capacity of the cellular network, without requiring that additional SIDs be provided. This is made possible in conventional systems by using certain information that is transmitted to the mobile station with the above-mentioned OMT. Of the transmitted information, of most concern to the teaching of this invention are information units referred to as the REGID and REGINCR.
When a mobile station registers with the system it stores a value for a variable referred to as NXTREG (i.e., next registration), where the stored value of NXTREG=REGID+REGINCR as received from the OMT. By example, if the mobile station has registered into the system with REGID=1000 and REGINCR=450, it stores the value 1450 into the NXTREG memory location. The system subsequently increments REGID from 1000 to, by example, 1002, 1004, 1006, . . . , etc. When REGID reaches 1450 the mobile station registers again, and stores the NXTREG value as NXTREG=(1450+450)=1900. This process continues, with the mobile station registering every time that the received value of REGID equals or exceeds the internally maintained value of NXTREG.
In conventional practice, and when entering the system access task to perform registration, the mobile station scans its receiver to identify the two strongest received access channels. The mobile station then registers into the paging area having the strongest access channel, from which it can read data sent on an access channel. If the mobile station cannot read and decode the data successfully, it switches to the second strongest channel and tries to complete the task there.
After a successful registration, and according to conventional practice (e.g., EIA/TIA-553), the mobile station does not stay on or continue to monitor the last-identified access channel, but instead begins scanning for a new "strongest" paging channel. Depending on the location of mobile station, and provided that the paging and access channels (P/A) are the same, the new "strongest" paging channel may be the same channel, or another channel from the same paging area, or a channel from another paging area altogether.
This latter case is troublesome, since if the "strongest" channel is from a different paging area than the paging area into which the mobile station has just registered, and assuming that the mobile station is able to identify control channel signalling from the most-recently determined strongest paging channel, the mobile station enters the idle state and begins monitoring the paging channel of the "wrong" paging area. In this case, and if the mobile station is called, the mobile station is paged in the paging area into which the mobile station last registered. However, if the mobile station is instead now tuned to the paging channel of another paging area, the mobile station will not respond to the paging message that is directed to the mobile station.
Furthermore, if the mobile station enters the idle state or task in another paging area than the one within which it started the registration process, it will detect that REGID is significantly different and will begin the registration process again.
A most detrimental problem resulting from the foregoing situation is as follows. The mobile station begins the registration process from a paging area 1 and the NXTREG is calculated from the information received from the paging channel of the paging area 1. The mobile station scans for the strongest access channel and finds it from the paging area 2. The mobile station then performs registration on the access channel from paging area 2. During system access the mobile station is not specified to read the REGID and, in most cases, it would not be available during the short period used for system access. When the mobile station scans for the paging channel after system access, it may again find a channel from paging area 1. When the mobile station compares the NXTREG to the REGID, it believes that there is no reason to perform registration. However, the system believes the mobile station is listening on one of the paging area 2 channels and sends pages there. As a result, the mobile station may miss a page that is directed to it from the system.
Furthermore, while the mobile station is scanning for other "strongest" paging channels, there exists a possibility that the mobile station will miss a paging message that is addressed to it.
This problem arises most often when a mobile station is located between or along the boundary of two paging areas where different REGIDs are used. In this situation the mobile station may be able to receive approximately equally strong signals from both paging areas.
This situation could be improved by sending another page in contiguous paging areas if the mobile station does not respond to the first page. However, this approach would cause additional loading of the system, with a consequent reduction in capacity.